1. Field of The Invention
This invention is directed to antenna systems in general, and to systems which are used with an automatic tracking antenna, in order to reduce or improve the signal-to-interference ratio, in particular.
2. Prior Art
There are many known uses for datalinks between one or more users of a particular system. The datalink is used to provide a transfer of information from a source to a receiver. The datalink may utilize microwave or optical wavelength electromagnetic waves. It is usual to provide an antenna at both the source and the receiver ends of the datalink. In this case, the antennas can act as radiators and/or receivers.
In a usual application, the antennas are arranged to point at each other and to produce directive reception and transmission of electromagnetic energy along a path between the respective units. If one or both antennas are mounted on moving vehicles, it is necessary to use some sort of automatic angle tracking or antenna pointing system to keep the antennas continuously pointed in the proper direction.
Unfortunately, in certain types of datalinks, especially those between airborne and ground units, unwanted interference emanating from various radars or different datalinks can often be encountered and picked up by the datalink receiving antenna. Moreover, because of the movement of the airborne vehicle relative to the ground receiving antenna station, the angle tracking must be quite accurate in order to avoid loss of signal strength due to antenna mispointing. Because of the interference which can be picked up, the antenna angle tracking can sometimes become noisy or the receiving antenna may lose tracking "lock" wherein the datalink is severed and the desired signal is lost.
In addition, in the airborne/ground station operation, it can frequently occur that a separate airborne vehicle (or aircraft) can fly into or through the area of the down-link arrangement. This second aircraft can, and usually does, include its own radar system or, possibly, its own down-link system. In any event, it frequently occurs that the ground station (the basic tracking station) will detect the second radar system and begin to track the secondary aircraft. Through this action, the down-link can be broken and lost. This situation can develop even though the secondary aircraft may be, in fact, a "friendly" aircraft. In some situations, such as a warfare situation, or a commercial operation comprising numerous aircraft in the same general area, the loss of the down-link contact can be very important and detrimental.